Refrigeration systems or chiller systems typically include a motor driven compressor, condenser, and evaporator in a closed refrigeration loop. Variable speed drives (VSDs) are used in many instances to power the compressor motor. The voltage and frequency of the AC voltage is varied at the output of the VSD to vary the speed of the compressor motor. In large compressors for use in commercial, industrial and large HVAC&R systems in particular, the compressor manufacturer typically sets forth a minimum rotational frequency of the compressor. This limitation is established by the manufacturer primarily to ensure sufficient lubrication for bearings in the compressor in addition to other considerations. The bearings and other moving parts are lubricated by oil circulating within the compressor. As the compressor speed decreases, the amount of oil available for lubrication is reduced.
In order to prevent damage to the compressor bearings and other moving parts due to insufficient lubrication, the manufacturers normally set forth a minimum operating rotational frequency for each compressor. This minimum operating rotational frequency, also referred to as the nominal minimum compressor frequency, is normally monitored by the chiller control system. As a precautionary measure, the control system shuts down the compressor in response to sensing the operating frequency drop below the nominal minimum frequency. This limitation can be problematic, for example, where the compressor speed is controllably reduced due to a temporary reduction in cooling demand. In some instances, the compressor frequency may drop below the nominal minimum frequency for only a brief period, or the compressor frequency may be controlled at a frequency below the nominal minimum frequency by a very small margin. In such a case, the risk of overheating the bearings due to a lack of lubrication is generally minimal, and the inconvenience and expense of restarting the chiller system outweighs the potential benefit of the system shutdown.
Therefore, there is a need for a control system that controls the actual operating frequency of the compressor below the minimum operating rotational frequency for a short period before turning the compressor off.